A silicon carbide semiconductor has a large band gap as compared to a silicon semiconductor and therefore, has high insulation breakdown field intensity. In the silicon carbide semiconductor, on-resistance is resistance in a conductive state and is inversely proportional to the cube of the insulation breakdown field intensity. Therefore, for example, widely used silicon carbide semiconductors called a 4H type can suppress the on-resistance to a few hundredths as compared to silicon semiconductors. A semiconductor element using a silicon carbide semiconductor also has high thermal conductivity characteristics facilitating heat radiation and therefore, is expected to be a next-generation, low-loss power device.
Power semiconductor elements generally have a vertical structure in which current is applied from the front side toward the backside. This is mainly because the proportion of an electrode pad, which is an element, to the total area is not negligible and the same current can be realized with a smaller area by forming one electrode on a wafer back surface. A back surface electrode is bonded through a solder layer to a copper (Cu) plate called direct bonded copper (DBC). On the other hand, a front surface electrode is wire-bonded to one end of an aluminum (Al) wire by ultrasonic waves and the other end of the Al wire is bonded to the DBC.
Two main capabilities are required of the back surface electrode of the power semiconductor element as described above. One is to reduce ohmic contact resistance at a metal/semiconductor interface and the other is to increase adhesive strength with the solder layer. For lower ohmic contact resistance, it is known that after deposition of nickel (Ni) on the wafer back surface, a nickel silicide (Ni2Si) layer may be formed by sintering at a temperature equal to or greater than 900 degrees C. in a reduction atmosphere. By forming the Ni2Si layer, a favorable value of 10−7 Ωcm2 is obtained as the contact resistance at a substrate (wafer) concentration of 1019 cm−3 (see, e.g., Patent Document 1).
However, in the method described above, as can be seen from Equation (1), carbon with a graphite structure is formed on the top surface. A portion of this graphite is precipitated on the Ni2Si layer surface and stable in terms of energy.SiC+2Ni→Ni2Si+C  (1)
FIG. 2 is a schematic of layer structure at a stage when an ohmic layer is formed by sintering after deposition of Ni on an SiC substrate. As depicted in FIG. 2, at the stage when the ohmic layer is formed, a structure of a graphite layer/an Ni2Si layer/the SiC substrate is formed from the surface side (exposed surface side) of the ohmic layer. For bonding with solder, for example, a multilayer film obtained by sequentially depositing titanium (Ti), Ni, and gold (Au) is formed on this ohmic layer, and it is known that the peeling of the multilayer film occurs due to the graphite layer precipitated on the top surface of the ohmic layer (see. e.g., Patent Document 2).
In a method proposed as one measure for suppressing the peeling of the multilayer film due to the graphite layer, after depositing not only Ni but also Ti on the SiC substrate, the ohmic layer is formed by performing the sintering to obtain the ohmic contact with the silicon carbide semiconductor (for forming the ohmic layer) and the multilayer film is then formed on the ohmic layer (see, e.g., Patent Document 1). Ni has reaction enthalpy with Si lower than carbon (C), and Ti has reaction enthalpy with C lower Si. Therefore, as represented by Equation (2), carbon released from SiC is converted by reaction with Ti into TiC.SiC+2Ni+Ti→Ni2Si+TiC  (2)
FIG. 3 is a schematic of layer structure at the stage when an ohmic layer is formed by sintering after deposition of Ni and Ti on an SiC substrate. At the stage when the ohmic layer is formed in this case, the layer structure from the surface side of the ohmic layer is a TiC layer/an Ni2Si layer/the SiC substrate, as depicted in FIG. 3 (see. e.g., Patent Document 3). For example, by forming a multilayer film obtained by sequentially depositing Ti, Ni, and Au on this TiC layer, adhesiveness between the ohmic layer and the multilayer film becomes very good.
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-344588
Non-Patent Literature 1: S. Tanimoto, et al, Materials Science Forum, Vols. 389-393 (2002), p. 879
Non-Patent Literature 2: S. Tanimoto, et al, Phys. Status SolidiA 206, No. 10, pp. 2417-2430 (2009)
Non-Patent Literature 3: M. Levit, et al, J. Appl. Phys., Vol. 80, No. 1 (1996) pp. 167-173